1. Field of the Invention
This invention relates to radiation curable compositions and printing inks for the manufacture of packaging materials. More particularly, this invention relates to solvent resistant radiation curable aqueous compositions and aqueous printing inks.
2. Description of Related Art
Energy curable, low viscosity inks and coatings are typically composed of mixtures of acrylated oligomers and monomers. Typically monomers are used to control viscosity of ink or coating formulations for flexographic, gravure, roller and tower printing and coating applications. However, diluent monomers do not react completely during polymerization upon exposure to ultraviolet (UV) or electron beam (EB) radiation. Such unreacted monomers remain as residual components in the dried printing ink or coating films and are subject to migration by absorption as well as surface contact. This migration of residual components leads to a host of problems, particularly for printing or coating “odor” and “off-taste” sensitive packaging for packages such as containers for tobacco, perfume, etc. In addition, sometimes solvents are employed to achieve a coating of lower viscosity.
An example of a solvent based coating is described in U.S. Pat. No. 5,824,717, Merill et al., discloses peroxide and radiation (energy) curable compositions containing isobutylene copolymers having acrylate functionality, and optionally a filler. The disclosed copolymers are acrylate modified copolymers of an iso-olefin of 4 to 7 carbon atoms and para-alkylstyrene co-monomers. Merrill discloses that the percentage of extractables from the cured composition is negligible, and that the cured composition is suitable for use in the manufacture of a variety of high purity rubber goods used in the pharmaceutical and health care industries. Merrill further discloses that the compositions may be used as condenser packings, food contact material, wire cable insulation materials, and in the manufacture of high purity hoses. Merrill discloses that coatings are prepared by dissolving the copolymer in toluene.
Problems resulting from odor, off-taste and residual extractables with currently available UV/EB printing inks and coatings has kept energy curable products at bay from the high volume packaging market, which still is largely served by conventional solvent or water based flexo printing inks and coatings which require the removal of the solvent or water before curing. Acrylated oligomers typically have viscosities which are too high to be used per se (i.e., without a monomer diluent) for making low viscosity coatings and especially printing inks.
The use of water as a diluent for mixtures of UV/EB curable acrylated oligomers is disclosed in U.S. Pat. No. 6,011,078 for wood and floor coating applications. The formulations are dispersions or emulsions, which require prior evaporation or imbition of water on non-absorbent substrates before exposure to light.
Thus, the present state of producing low extractable packaging materials, which are not as low as to be FDA compliant for direct food containers, requires the use of solvents and/or dispersions using a multi-step procedure wherein evaporation or water or non-absorbent substrates is necessary.
There continues to be a need for homogeneous, monomer and solvent free, UV/EB curable aqueous formulations which provide high solvent resistance and have insignificant odor and/or low levels of extractable components.